If you look into the technical resource section of Building Science Corp., neither is correct; their call-out is to use no vapor barrier. The thinking basically goes this way: Even the best-built basements potentially will leak moisture through the walls, either as liquid but also as water vapor. A vapor barrier will only serve to trap this moisture and create potential mold/mildew problems - potential health issues. The proper way to deal with this potential source of water is to limit the water entry through the walls and then let any that passes dry to the interior with vapor permeable materials of construction. That means no visqueen. One OK method is to insulate with white beadboard foam insulation and then drywall the interior. Fiberglass insulation isn't recommended because it holds moisture. Here is a link that might be useful: Building Science Corp. - Basement Insulation Systems

Moisture is generally driven into wall systems by moving from the warm side of the wall to the cooler side of the wall. In northern or heating climates, vapor barriers are put on the interior side of the wall, normally right behind the drywall. The goal is to prevent interior moisture vapor from entering into the stud cavity and in cold weather, condensing within the wall. In northern climes housewraps, such as Tyvek, which are designed to shed liquid moisture but allow moisture vapor to pass through, are commonly installed on the exterior of the wall, OVER the sheathing, but UNDER the siding. Tyvek allows moisture tha has gotten into the wall to dry to the outside, yet it prevents liquid moisture (wind-driven rain, solar driven moisture) that gets behind the siding from getting into the wall system. In southern or cooling climates, the vapor barrier is normally put on the exterior side of the house. This prevents outside warm humid air from being driven into the wall system toward the interior cooler air conditioned space. WEre humid air allowed into the wall system, as it cooled due to the cool indoor air conditioned air, moisture vapor could condense within the wall's stud cavities. Or something like that! In Florida, when plywood sheathing (or something like T111 siding which it sounds like you are using) is being used on the outside of the wall, an additional vapor retarder/barrier is generally not required. Some southern builders will still use Tyvek/Typar or some other type of housewrap between the framing and the siding, but remember, these housewraps are NOT vapor barriers. They are vapor permeable, but will still shed water droplets. Mongo

So what are you having as the FINAL floor? It sounds like you are installing 1/2" cork FLOORING TILES....but then a little further on you are talking about wood over cork. This tells me you are talking about cork UNDERLAYMENT underneath hardwood. OK. Now we are on to something. A vapour barrier is required if you are AT GROUND LEVEL or BELOW. Your installer can glue down the cork to the SUSPENDED SLAB (aka....10th floor concrete slab) without worrying about a vapour barrier. A sheet vapour barrier is NEVER USED with glue down anything. The glue must act as the vapour barrier. Glue down hardwood over glue down cork underlayment is a MAGNIFICENT floor. And it is one of the MOST EXPENSIVE installation formats for hardwood that we have (yep....it is more expensive than you can imagine). If your engineered wood can be FLOATED then you will save HUGE amounts of money (could be as much as $7 - $10/sf worth of savings). The cork can be floated (it just sits there....no glue) and then the wood is floated over that (glued at the edges and tapped together). It is a MUCH faster installation (can be 2 days faster). But first: what floor do you live on?

The perm rating for exterior-grade plywood is what it is despite any physical defects typical in the material. While it has a low perm rating, it's a poor candidate to install under sheetrock as a vapor barrier. In my market, 1/2" CDX plywood costs roughly $.55/SF compared with 6 mil polyethylene sheeting which costs $.05/SF .... @Charles Ross, looking back at the picture I posted the plywood has a perm rating of .70 and the polyethylene sheeting has .06 --- the picture states anything with a rating less than 1 qualifies as a 'vapor barrier'. So the question of which is better I'm not really arguing, just expanding the conversation to show that there are different materials to use as a vapor barrier as well as the location of which to install it (climate concerns). With that said, what is better in terms of the rating regardless of cost or installation problems or concerns? Here in the south they not only put up plywood but then also wrap the house, usually in Tyvek. But the home I live in was built in the late 70's has either asphalt coated paper or some sort of aluminum coated paper. Another important difference is in my area the product is installed on the exterior of the house. I think a wrap would be a harder install on interior walls. I think if it were me I would choose the 'Vapor barrier paint or primer' for homes in the north where vapor barrier should be installed internally. The rating of the 'vapor barrier paint or primer' according to the picture I posted a few clicks above was rated at 0.45 which is almost half of plywood... but much higher than polyethylene at .06. Due to the ease in which the 'vapor barrier paint or primer' can be applied, the higher rating and no interference that the ply wood make due to the thickness of it, 'cracks and crevice' problem I mentioned... But the question is as someone who doesn't do this sort of thing... what is the difference of the rating between these different materials.... short of costs, installation methods or problems with such. Is a rating closer to 1 better than a rating closer to 0? That primarily is the point to everything I posted, it wasn't a venture off into the weeds. (What's the best in terms of the rating?)